During nuclear reactor operations, fuel cladding defects inevitably cause some fission product krypton and xenon gases to escape from the reactor fuel and become distributed throughout the various reactor gaseous effluents; the quantity and composition of these distributed gases depending upon the type of reactor utilized, the quality of the fuel, and the irradiation time. Inasmuch as these fission product gases are not completely soluble in the reactor coolant, some portions of the gas are available for release to the atmosphere. Thus, in order to limit the hazards of radiation exposure posed by these fission products to the public, various off-gas treatment systems for removing these products until their decay have been installed at nuclear power plants.
In boiling water reactor (BWR) systems, radiolysis of the cooling water in the reactor vessel generates gaseous hydrogen and oxygen. These gases, together with the fission product noble gases escaping from the fuel, are carried by process steam to the reactor's main condenser. This moisture laden gas mixture is then removed therefrom by a stream jet air ejector (SJAE) and directed to the off-gas effluents treatment system, such treatment systems, as discussed in R. J. Tossetti, "Gaseous Waste Treatment," Chapter 5, Proposed American National Standard Gaseous Radioactive Waste Processing Systems for Light Water Reactor Plants, ANS-55.8, N720, Draft 10, January, 1978, ranging from simple storage to cryogenic distillation.
All current BWR off-gas treatment systems have three common functions. First, hydrogen and oxygen from the SJAE exhaust are removed from the process steam by passing these gases through a catalytic recombiner, followed thereafter by removal of the process steam from the remaining off-gas effluents. Next, remaining residual water vapor is removed by a desiccant dryer or freeze-out exchanger.
Finally, most of the fission product gases present in the effluent stream are treated by separation therefrom, and then either stored in holding tanks to ensure decay of most of the radioactive components or delayed for a time sufficient to allow for decay to innocuous levels prior to atmospheric discharge.
Recombination of hydrogen and oxygen typically occurs in the presence of platinum/palladium-type catalysts with either a metallic or ceramic base, the off-gas hydrogen being diluted to keep its concentration at or below 4% by volume, the hydrogen hazardous limit in air, and to prevent excessive post-reaction temperatures within the recombiner unit.
The removal of residual moisture from the effluent stream, a desirable step in order that the adsorptive efficiency of the treatment system towards the fission product gases be increased, is achieved by passing the stream throught a parallel series of molecular sieve beds. When saturation of the on-line bed occurs, the flow is directed to an alternate bed and the saturated bed regenerated by passing heated air therethrough to desorb the retained moisture.
The final phase of a conventional treatment process comprises the use of charcoal beds to effect the removal of krypton and xenon from the effluent stream. The required hold up time (i.e., removal efficiency) of a charcoal bed is a function of gas flow rate, charcoal mass, and dynamic adsorption coefficient, approximated by the equation: EQU T=0.53(MK/F)
where,
T=hold up time in hours PA0 M=mass of charcoal adsorbent in tons PA0 K=dynamic adsorption coefficient in cm.sup.2 /gm, and PA0 F=gas flow rate in scfm.
Since dynamic adsorption coefficient inversely relates to operation temperature, various cooling system configurations are incorporated into effluent treatment system designs so as to reduce the quantity and volume of the charcoal adsorbent beds; however, such use is at the expense of significant increases in construction costs for the treatment system.
Unlike BWRs which utilize process steam to remove waste gases generated by reactor operations, pressurized water reactor (PWR) gaseous effluents are collected and removed by various vents and tanks within the reactor vessel, such vents and tanks including the gas stripper, reactor coolant, drain tank vent, equipment drain tank vent, chemical and volume control system hold up tank vent, and the volume control tank purge. Hydrogen gas generally represents approximately 70% of the total off-gas effluent stream.
The treatment system presently used in most PWRs is the waste gas decay tank system wherein fission product gases, collected from the several tanks and vents, are passed through a manifold prior to entry through a surge tank to allow for noncontinuous flow of the influent gas. The gas is subsequently directed to a compressor, the gas having previously been filtered to prevent particulates from building up in the compressor and causing damage. The compressed gas is then stored in one of several large decay tanks. Normally, one tank is being filled while two or more tanks are decaying in isolation and another tank is depressurized by releasing its contents to the atmosphere. The number and size of the decay tanks used is dependent upon several factors, the primary factor being the hold up time of the waste gas, typically 30 days, necessary to achieve the maximum permissible site boundary radioactive hazard limit.
An alternate method for PWR effluent treatment is by effecting the selective adsorption of fission product gases by means of an adsorbent charcoal bed. The effluent stream is first passed through an after-cooler to remove the heat of compression and lower the absolute humidity of the gas in order to ensure proper charcoal adsorbent bed performance. For gas flow rates typically associated with a PWR, approximately 3,000 pounds of charcoal are required for 30 days of xenon hold up.
The discussion thus far assumes, of course, normal system operation and circulation of cooling water throughout the reactor vessel. However, in the event of a loss-of-coolant accident such as that which recently occured at Three Mile Island (TMI-2), the loss of coolant and resultant heat rise in the fuel elements due to radioactive decay, may result in the breakdown of fuel integrity and subsequent rapid rate discharge of volatile and semi-volatile fission products. In such an overheated condition, zircaloy fuel cladding material reacts with any water present in the reactor or containment vessel to form hydrogen gas, several hundred cubic feet of which can be produced within a relatively short period of time. Additionally, the use of alkaline sprays to cool down the contaminated atmosphere within a containment vessel will also generate hydrogen gas due to reaction with various metallic parts therein.
The prior art has handled the problem of explosive risk presented by hydrogen gas formation during normal reactor operations by incorporating hydrogen recombiners into some of the various off-gas treatment systems designs, a not altogether satisfactory solution as a significant number of hydrogen explosions have occured as a result of such hydrogen-oxygen recombination step.
In the event of a reactor system failure, containment vessels are usually provided with either emergency hydrogen recombiners or venting arrangements for connecting the off-gas treatment system recombiner to the containment atmosphere whereby to remove hydrogen gas present therein.
Though the prior art has recognized the danger associated with hydrogen formation, none of the various reactor designs presently in use have included provision for controlling the venting of containment vessel atmospheres whereby to prevent the atmospheric release of radioactive noble gases generated as the result of a system failure.
Therefore, it is an object of the present invention to provide an integrated off-gas treatment system adaptable for treating either off-gas effluents produced during normal reactor operations or combustible and fission product gases released as the result of a nuclear reactor system failure.
A still further object is to effect the separation of hydrogen from the off-gas in a simple and safe manner which eliminates the need for a hydrogen recombiner, thereby reducing both the risk of hydrogen explosion and the overall cost of the off-gas treatment system.
Another object is to provide an off-gas treatment system adaptable for use with BWR, PWR, and other types of nuclear reactors.
And yet another object is to provide an off-gas treatment system wherein the majority of treatment steps are performed at or near ambient temperature and pressure.